The present invention relates to self-powered neutron detectors and to detector assemblies incorporating a plurality of detectors for in-core nuclear reactor radiation monitoring. A self-powered detector is a device which does not require a voltage potential, but generates a signal between a central conductive emitter and a spaced coaxial sheath collector, with insulation means provided therebetween. A large number of such detectors as required to adequately monitor the radiation levels throughout the reactor core to provide sufficient data to characterize the core fuel performance and to provide safety monitoring data. The detectors are typically spaced along the length of the core to provide neutron flux measurements along the core length.
In the presently used multi-detector assemblies, the active detector portions of the individual detectors are spaced along a longtudinal path of the bundled assembly so that the cable leads for the respective spaced detectors only extend as far as the respective detector active portion. This means that several cables are closely spaced from the first detector while none are present at the furthest extending terminal detector of the assembly. This difference in mechanical structure and materials present in the immediate area of each detector active portion results in variations in detector sensitivity due to the individual local perturbation factors, and complicates analysis of the signal outputs from the various detectors.
A neutron detector assembly is described in U.S. Pat. No. 3,751,333, in which the detectors and their lead cables are disposed between a hollow center calibration tube, and a laterally flexible outer sheath, with mechanical spacers running between the spaced detectors and cables along the entire length of the assembly. The spacers keep the detectors properly spaced. In the commerical design of the above-described patent, the spacers are neutron absorptive solid wire members which depress the neutron level at the active detector. This is particularly the case at the terminal end of the assembly where a plurality of such wire spacers is in close proximity to the furthest extending active detector. This terminal active detector sees a lower neutron flux for a given incident flux level than the other spaced detectors where there are less solid wire spacers.